Elsevier

Carbon

Volume 185, 15 November 2021, Pages 17-26
Carbon

Research Paper
3D MXene anchored carbon nanotube as bifunctional and durable oxygen catalysts for Zn–air batteries

https://doi.org/10.1016/j.carbon.2021.09.004Get rights and content

Highlights

  • FeCo alloy nanoparticles encapsulated N-doped carbon nanotubes (CNT) have been anchored on the surface of N-doped Mxene.

  • The 3D conductive network exhibited excellent ORR performance.

  • The Zn-air battery showed 375h discharge and cycle performance.

Abstract

The development of bifunctional activity and long durability of ORR/OER catalyst is vital for realizing the practical application of Zn–air batteries (ZAB). Herein, 3D-structured bifunctional catalysts, CoFe nanoalloy decorated carbon nanotubes (CNT) anchoring with 2D MXene (Fe/Co-CNT@MXene-T), were prepared and fabricated as the cathode for both aqueous and flexible ZAB. Taking advantages of active sites of CoFe nanoparticles, robust sheath and fast charge transfer provided by CNT and MXene, our optimal catalyst Fe/Co-CNT@MXene-8 exhibited remarkable electroactivities of ORR and OER comparable with the benchmark Platinum carbon (Pt/C) and Ruthenium dioxide (RuO2), respectively. The as-assembled aqueous ZAB with Fe/Co-CNT@MXene-8 as cathode exhibited a high specific capacity of 759 mA h g −1 at a current density of 10 mA cm−2, and a stable voltage platform in the quick discharge–charge test under a 375h long-lasting operation, superior to those assembled with benchmark commercial catalyst and other reported catalysts. Furthermore, the as-assembled flexible ZAB with Fe/Co-CNT@MXene-8 as cathode also presented excellent durability cycling and showed promising perspectives as future power sources for wearable application. Such high performance was also verified by X-ray absorption near-edge structure (XANES) and supported by density functional theory (DFT) calculation. This work provides a novel view to design highly effective and long-lasting rechargeable ZABs.

Graphical abstract

FeCo alloy nanoparticles encapsulated N-doped carbon nanotubes (CNT) have been successfully anchored on the surface of N-doped MXene.

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Introduction

Developing efficient and sustainable energy conversion device is a promising strategy to tackle the current energy and climate challenge. Rechargeable Zn–air batteries (ZABs) are known as one of the candidate devices due to their high theoretical energy density and abundance of zinc metal sources [[1], [2], [3]]. However, the sluggish kinetics at the cathodes of ZABs lead to high thermodynamic overpotentials and low output power, limiting their practical applications. Expensive precious metal and its oxide are the specifically electrocatalysts currently used as cathodes materials of ZABs, thus it is demanding to improve the ZABs towards high efficiency and low cost [4].

Studies have been reported to combine transition metal nanoparticles (e.g. Co, Fe, Ni, Cu) with high conductive material (e.g. graphene nanosheet, transition metal carbides) [[5], [6], [7]], such as bimetal alloy coated in N-doped carbon nanotubes (CNTs) in suit growth with conductive material and FeP/Fe2O3 nanoparticles incorporated N, P-doped carbon aerogel [8]. However, the research into the construction of complex structures using such combinations for high-performance ZABs is rare [3].

MXenes, a new of two-dimensional (2D) transition metal carbides or nitrides exfoliated by MAX, are receiving increasing research attention in the energy conversion devices due to their essential electrochemistry properties [[9], [10], [11], [12], [13]]. For example, pristine Ti3C2Tx MXene film has been reported to exhibit attractive properties [[14], [15], [16], [17]], and MXene electrode exhibits an ultrahigh electronic transmission capability [18,19]. However, it is still far from satisfactory for its application of ZABs, because the restacking of the Ti3C2Tx layers always occur, which needs insertion processes to extend electronic transmission path [20,21]. Herein, we thought designing unique 3D MXene conductive structure could be one of the ways to deal with aggregation problem of such 2D nanosheets [[22], [23], [24], [25]].

In this work, we developed a one-pot pyrolysis method to prepare novel ZABs catalysts in 3D structure. FeCo alloy nanoparticles encapsulated N-doped carbon nanotubes (CNT) have been successfully anchored on the surface of N-doped MXene (Fe/Co-CNT@MXene-T). The resultant Fe/Co-CNT@MXene-8 catalyst exhibited outstanding electrocatalytic activities and high stability towards both ORR and OER. The bifunctional activity voltage for OER and ORR is as high as 0.73 V (vs.RHE), which surpasses precious metal benchmark Pt/C + RuO2 and most of other nonprecious metal catalysts. Fe/Co-CNT@MXene-8 also exhibited a long cyclability as a cathode in both liquid and solid ZAB systems. It could retain a remarkable stability and present a high-power density of 150 mWcm−2 during a 375 h cyclability test.

It is believed such high performances of ZABs could be explained by the following reasons: (1) The unique 3D structure consisting of 1D CNT and 2D MXene ensures the fluent charge transport; (2) The bamboo-like CNT with large surface area not only can exposes more active sites, but also speed up the electrolytes streaming and oxygen molecules flow; (3) The N dopant onto the metal surface improves surface defects and potential energy; (4) The synergistic effect of active sites FeCo nanoparticles improves the catalytic activity.

Section snippets

Materials

All the materials were used without any additional purification. Polyethyleneimine (PEI, MW = 600), cobalt nitrate hexahydrate (Co(NO3)2.6H2O), melamine, ferric chloride hexahydrate (FeCl3⸱6H2O), zinc acetate (Zn(CH3COO)2), MAX (Ti3AlC2), lithium fluoride (LiF), hydrochloric acid (HCl, 37.5 wt%), potassium hydroxide (KOH), Commercial Pt/C (Pt, 20 wt%) and RuO2. Nafion solution (5 wt%). The deionized water (18.25 MΩ)” was replaced by “All the materials were used without any additional

Results and discussion

The preparation of Fe/Co-CNT@MXene-T catalyst is schematically illustrated in Fig. 1. The MXene (Ti3C2Tx) nanosheets were first produced by delaminating MAX (Ti3AlC2) with HCl/LiF under ultrasonic treatment [[26], [27], [28]](Fig. 2a). MAX phase has a micron-size block structure (Fig. S1a). The XRD shift at the peak of 10° to a smaller angle and the greatly weakened peak at 39° support the formation of MXene (Fig. S1b). The AFM results show that the prepared MXene are multilayer, with an

Conclusions

A 3D-structured bifunctional ORR/OER catalyst, CoFe nanoalloy decorated CNT anchoring with 2D MXene, has been successfully synthesized. The ZAB equipped with our Fe-Co/CNT@MXene-8 cathode exhibited a high energy and power density, stable voltage output, and remarkable stability in both aqueous and solid system. One reason for such high performance and stability can be contributed to the 3D structure. The CNT reorganize the alignment of the Ti3C2 flakes, making the electronic transport pathways

CRediT authorship contribution statement

Chao Zhang: Conceptualization, Methodology. Huifeng Dong: Data curation. Binling Chen: Writing – review & editing. Tianxu Jin: Data curation. Jun Nie: Supervision. Guiping Ma: Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Grant No. 51973009). The project was funded by State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Dong Hua University. (KF2105)

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